1. Field of the Invention
The present invention relates to a FET amplifier, and more particularly to a FET amplifier capable of minimizing the degradation of distortion in the output power due to temperature variations in the operating ambience.
2. Description of the Related Prior Art
As a device for use in a high-output amplifier in microwave band communication apparatuses, an LDMOS FET (lateral diffused MOS field effect transistor) is being increasingly adopted. This device has an advantage of having better output distortion characteristics than a GaAs FET (gallium arsenide field effect transistor) which can generally provide a high output more easily. For this reason, it is suitable for use in wireless base stations of the W-CDMA (Wideband Coded Division Multiple Access) type. At the same time, this device has a disadvantage that its operating current (drain current) greatly varies with the ambient temperature of operation and its susceptibility to distortion also greatly varies with the fluctuations of this drain current.
Even if the drain current is set to a certain amperage at the normal temperature, the amperage of the drain current will vary if the ambient temperature varies. As a result, a wide difference may arise between the consumed amperage at the normal temperature and that at a lower or higher temperature, and this would be undesirable for reducing power consumption by the system.
As the distortion-susceptibility of a LDMOS FET is highly sensitive to the drain current and its sweet spot is narrow, it may greatly vary with an up or down in ambient temperature. From the viewpoint of suppressing the distortion-susceptibility, in order to use an LDMOS FET in a broad temperature range, it is necessary either to choose a device having a higher output capacity than is required or to perform temperature compensation with an external circuit.
Thus, for an amplifier using an LDMOS FET, temperature compensation of the gate voltage is an indispensable technique from the viewpoint of reducing its power consumption, size and cost. In order to maintain the distortion-susceptibility of an LDMOS FET amplifier at the optimum point within the ambient temperature range of operation, it is indispensable to control the gate voltage Vgs so as to keep the drain current Ids constant.
An example of the prior art to compensate this gate voltage of the FET is disclosed in the Japanese Patent Application Laid-Open No. Sho 57-157606. FIG. 1 illustrates one example of external temperature-compensating circuit according to the prior art.
The circuit of the prior art shown in FIG. 1 includes a thermistor 54 in part of a voltage dividing circuit for supplying the gate voltage, and is so configured as to subject the gate voltage Vgs, to be applied to a FET 51, to temperature compensation by utilizing temperature-dependent variations in the resistance of the thermistor.
The temperature range permitting linear compensation of the gate voltage Vgs in this temperature-compensating circuit is around 30° C. If the ambient temperature is supposed to range from −10° C. to +80° C. in view of the likely application to communication apparatuses, it is difficult to compensate the gate voltage in a broad temperature range with the temperature-compensating circuit shown in FIG. 1 because the temperature-dependent variations in the resistance of the thermistor in this temperature range are by at most two digits.
An example of the prior art permitting temperature compensation in a broad range is disclosed in the Japanese Patent Application Laid-Open No. Hei 4-317205. FIG. 2 illustrates another example of LDMOS FET-based temperature-compensating circuit according to the prior art.
With reference to FIG. 2, gate voltage data matching the ambient temperature of operation are stored in a memory 63 in advance, and a controller 62 provides the output of a temperature sensor 61 to the memory 63 as an address signal and reads the gate voltage data out of the memory 63. A D/A converter 64 subjects to D/A conversion the gate voltage data that have been read out and applies the converted data to a FET 65 as the gate voltage.
However, this example of the prior art requires such circuits as an operational amplifier, controller and a memory among others in addition to the temperature sensor, and accordingly invites an increase in circuit dimensions and cost.